Related Stories

Today there are more than 140 million people and a trillion dollars in infrastructure in the first one metre above high tide level around the world.

Coastal areas (people, infrastructure and the environment) are already affected by extreme events such as coastal flooding and coastal erosion. For example, Hurricane Katrina caused over $100 billion of damage and the loss of over 1,000 lives along the USA Gulf coast and cyclone Nargis resulted in the loss of well over 100,000 lives in Burma.

And just last week, Cyclone Yasi caused widespread damage and storm surge flooding along north Queensland's coast.

These events remind us that coastal societies in both developed and developing nations are vulnerable to extreme coastal sea levels. This vulnerability will increase with rising sea level as a result of anthropogenic climate change.

With greenhouse gas emissions continuing to rise, it is becoming increasingly clear that modern coastal societies will need to adapt to rising sea levels. To cost effectively adapt, we need to know what to adapt to. How, why and where sea level has changed in the past, how it is changing now, and how it will change in the future.

Sea levels in the 21st century

Currently, more than 200 million people are already vulnerable to flooding by extreme sea levels around the globe. This population could grow to 800 million by the 2080s.

Over the last 140,000 years, sea level has varied over a range of more than 100 metres as ice sheets waxed and waned. While these changing sea levels allowed migration of early humans out of Africa and to distant continents such as Australia and North America, our modern society has only developed since climate stabilised after the end of the last ice age. During this period, and particularly over the last 2000 years, global averaged sea level has been relatively stable allowing society to develop coastal infrastructure close to present day sea level.

The rate of global averaged sea-level rise has increased from the 19th to 20th century and during the 20th century. Since late 1992 when we began observing global sea levels with satellite, sea level has been rising at over three millimetres a year — an order of magnitude larger than the pre-industrial period. While this rise is barely noticeable each year, it accumulates over time, like interest on an unpaid credit card debt. The impact is felt most acutely during extreme events such as storms and the associated surges at the coast or in estuaries.

Driving change

There are two principal reasons for rising sea levels. Firstly, the ocean is the Earth's thermometer — as the ocean warms as a result of increasing greenhouse gas concentrations in the atmosphere, it expands. Secondly, a warming atmosphere and ocean results in the melting of ice on land and the more rapid sliding of some glaciers into the ocean leading to a further increase of ocean volume.

Over the last 50 years, ocean thermal expansion has accounted for about 40 per cent of the observed rise with the melting of glaciers in places like Alaska and Patagonia contributing an additional 40 per cent. Changes in the great ice sheets of Greenland and Antarctica and terrestrial storage of water (the building of dams and the depletion of waters in aquifers) account for the remaining 20 per cent. The contributions from glaciers and ice sheets have increased over the last decade.

These processes will continue to contribute to rising sea levels during the 21st century and for centuries into the future. However, we know little about the deep ocean and the movement of ice sheets. Ongoing monitoring and improved understanding of their response to global warming is urgently required.

Without significant and urgent mitigation of greenhouse gas emissions, their concentration could cross the threshold that leads to a potentially irreversible melting of the Greenland ice sheet and metres of sea level rise. While melting of the Greenland ice sheet could take millennia, speeding up of the outlet glaciers may lead to a higher rate of rise this century.

Changes in mean sea level increase the frequency of coastal flooding events at spring tides and during storm surges, as well as increasing the potential for coastal erosion.

In many locations around the world there has been a significant increase in the frequency of high coastal sea levels. On both the east and west coasts of Australia there has been an increase in the frequency of high sea levels by about a factor of three during the 20th century. By 2100, the current 1-in-100 year coastal flooding event could be occurring more than once a year in many locations.

We need to adapt

Reducing our emissions of greenhouse gases can decrease the rates and amounts of rise but it is clear we cannot stop all sea-level rise. We will need to adapt.

There are three basic options available to adapt to rising sea levels. We have already adopted these options during the 20th century and will continue to use them during the 21st century.

Firstly, we can protect coastal populations and valuable coastal infrastructure, such as that in the low-lying Netherlands. The city of London has the Thames barrage which protects more than 100 billion pounds of assets. However, when these coastal defences fail in extreme events such as Hurricane Katrina, the impacts can be catastrophic.

Secondly, we can prepare for rising sea levels by changing the way we use the coastal region and designing appropriate infrastructure. Such infrastructure includes shelters where people can retreat to in times of severe storm surges and that have saved thousands of lives in Bangladesh.

Thirdly, it will be too expensive to protect all the world's coastlines and parts will be abandoned to the rising ocean, as is already being done in the UK.

Ongoing monitoring, improved projections and early warning systems of sea-level rise and extreme events will help adaptation and our responses to them.

Comments (37)

Comments for this story are now closed. If you would like to have your say on this story, please email ABC Science

Hawkeye :

08 Feb 2011 7:36:20pm

Please Dr Church, how can you justify your statement "rising sea level as a result of anthropogenic climate change"? The sea level has risen about 120 metres in the last 18,000 years, that is, since the end of the last ice age. That is how we know that we have moved into an interglacial age, the temperature rises, the ice melts and the sea level rises.

Geological research indicates that it happens on a cycle of about 100,000 years. It is all perfectly natural and nothing to do with man-made CO2 emissions.

An example of the fallacy is shown by the data from the International Energy Agency for 2007. Of 137 nation/states, the third greatest emitter per square kilometre was Hong Kong. The lowest emitter was Namibia. Hong Kong reports a temperature increase of 1.2 degrees C over 100 years. Windhoek, capital of Namibia, reports a temperature increase of 1.5 degrees C for emissions that are one 10,000th part of that in Hong Kong. That is 25% greater temperature increase for one 10,000th of the emissions.

As carbon is the fourth most abundant of the 92 naturally occuring elements, it is remarkable that so little is in the atmosphere because there is sure one heck of a lot in the rocks put there by plants and animals absorbing it from the atmosphere often via the ocean as you would know.

Kim Peart :

09 Feb 2011 9:12:11am

A greater problem than denial of basic science, is the scientific reticence that is unable to consider the full solutions that must be applied to save our hides. Anyone interested in undertaking the research can see that we were in a position to build solar power stations in space in the 1970s, to access the unlimited energy well of the Sun. This would have left most of the fossil carbon grounded. Instead, we released the carbon of dead life to gain energy and now we are in strife.

If our fork in the road was in the 1970s, then does a safe future on Earth lie in recapturing that lost opportunity, access the unlimited energy well of the Sun and be in a position to save our hides?

On the one hand we would have access to unlimited energy to deal with problems on Earth. On the other hand we will have secured a survival position beyond Earth, where heavy industry can be located.

There are many changes that we will not stop on Earth, like sea level rise, but using the muscle of space technology we can build our way through it and in time win back a safe Earth.

Why throw away the opportunity of becoming a star-faring civilization and winning back a healthy Earth, by clinging to the Earth and becoming increasingly unsustainable, until the moment arrives when our bubble simply bursts and we slide into the abyss of collapse.

timothyh :

Proof of an identical standard as above is also required that anthropogenic CO2 emissions have NOTHING to do with climate change. Warning: if you go to Google Scholar and use search terms based on the above issues, you will find humungous amounts of high quality proof, 99.9% of which will NOT support your viewpoint.

Finally, on what basis do you justify your out of context correlation between local CO2 emissions and local temp? Climate is much more complex than that!

Filz :

12 Feb 2011 1:54:50pm

timothyh - let's try and keep the argument as simple as possible (albeit in a highly technical subject). If sea levels are rising as alleged and if water always tends to find its own lever, why isn't the sea level where I live rising? Could it sometimes be that the sea level isn't rising, it's the land that's sinking?

Henry :

09 Feb 2011 12:17:03pm

Hawkeye,

CO2 doesn’t remain where it is released. It gets mixed into the atmosphere and distributed pretty evenly throughout the world. Your so called example of a “fallacy” only demonstrates your lack of understanding of even basic science principles.

But hey, what’s science got to do with it anyway? This conspiracy to create a world government has to be stopped!

arf :

The cycles of 100,000 years are quite natural. It is the trend that has commenced over the last two centuries that is not.*

Your 'proof of fallacy' makes a rather startling assumption: that carbon emitted in the atmosphere stays in one locality.

(*Ironically, it is possible that early man caused a slight global *cooling* in the last interstitial when he offed the mammoths and thereby created the frozen peat bogs of the tundra. And yes, that is only a hypothesis)

Dr Mat :

09 Feb 2011 1:26:24pm

Justification:

http://en.wikipedia.org/wiki/Current_sea_level_rise

The rate of rise has increased dramatically in the last few years. This is not natural.

In case you hadn't noticed, CO2 is a gas. It does not sit around in lumps where it was emitted. It diffuses rapidly around the world, and does not care where it was emitted, so the only relevant figure is the concentration in the entire global atmosphere.

John McLean :

I think I think :

15 Apr 2011 11:59:54am

That is just ridiculous John. Wikipedia is a perfectly suitable resource for anyone with common sense.

The article in question is particularly well referenced - the reader can choose to follow up the conclusions drawn if they so wish, and refute the claims made - if they have a credible argument. Do you?

Mike :

09 Feb 2011 4:37:33pm

Paragraph 2: If you want us to accept sea level changes are natural - you need to account for the observed changes, in terms of observed natural processes. You need to explain why, if sea level rise is natural, and its time scale is 100,000 years, its rate is suddenly 10 times up. Why have things suddenly changed rapidly? What processes in the world, account for this change?

paragraph 3: Carbon dioxide emitted by one place, spreads out and affects the entire planet. Hence, local temperature changes, arent much correlated with local carbon emissions.

Paragraph 4 - it's not clear what point you are trying to make. Is it that substances in low concentration can only have small effects? Other gases in low concentration have big effects. Ozone in minute quantities, protects us from UV. CFCs in minute quantity, raise UV levels, by destroying ozone. Thus there are 2 well known precedents for dilute gases, affecting conditions , for life on earth.

David Arthur :

the 120 m sea level rise to which you refer didn't happen SINCE the last glacial period ended, it was PART of the ending of the glacial period ("Ice Age", in the common parlance).

You see, when the glacial period ended, the ice sheets covering Siberia, Northern Europe and North America all melted, and the water ran off into the seas. This is what caused the sea level rise.

You write "Geological research indicates that it happens on a cycle of about 100,000 years. It is all perfectly natural and nothing to do with man-made CO2 emissions." Well, man-made CO2 emissions have COMPLETELY disrupted this 'natural cycle'. Matter of fact, the Little Ice Age of the 16th-19th centuries was actually the start of the next glacial period, but luckily, humans cut down enough CO2 sequestering forests to all but stop further cooling. Then, humans started burning fossil fuels as quickly as they could, and the entire colling process was thrown into reverse.

Ken :

10 Feb 2011 6:20:39am

So Hawkeye,all that research indicating anthropogenic influence on climate change is bull. I/m open to persuasion so could you and Andrew Bolte et al start providing links to the same number and quality of peer reviewed refutations of human induced climate change?? Please!!

lmwd :

12 Feb 2011 2:11:40pm

List of 850 publications critical of the theory of AGW. Link below. Oh, and Moderator, dare you to let this through! We wouldn't want to come away with the impression that the ABC are using their position to censor well supported contributions from the non-believers now, would we? http://www.populartechnology.net/2009/10/peer-reviewed-papers-supporting.html

Henry :

14 Feb 2011 12:26:47pm

Gee Lmwd, Some of those papers date back to 1989 and are discussing questions long since answered. But hey, never mind. If you are desperate enough to have to count out of date research in order to make up a number, who needs to actually read any of the papers!

A more balanced presentation is online at:http://www.skepticalscience.com/

Glenn :

ABC (Moderator):

10 Feb 2011 10:17:14am

Hi Glenn,Thanks for pointing out this error in the original text. The sentence has now been edited to read:"Over the last 140,000 years, sea level has varied over a range of 100 metres as ice sheets waxed and waned." More information can be found on the CSIRO website: http://www.cmar.csiro.au/sealevel/

Matt :

12 Feb 2011 12:01:42am

While I admire the simplicity of this solution I feel it may be a bit simplistic. It's not just housing that is the problem after all.

Infrastructure such as ports, electricity networks, manufacturing, agriculture and transport networks are expensive - and are paid for over a long time. Rebuilding infrastructure at higher elevations when you've not even paid for the existing infrastructure being ababdoned is enough to send a country to the wall.

The other problem of course is people having access to accurate, consistent information on which to make informed choices. From what I've seen so far this is a long way from happening if it ever happens at all.

naughtee :

12 Feb 2011 9:47:19am

unfortunately they'll most likely receive considerably tax assistance to re-build (on the same spot no doubt) when their beach side holiday houses are swept away ;)

however i think you'll find the real issue with building on or near dynamic coastal regions is by doing so you reduce the systems ability to resist or move with the erosion, this effect has been seen world wide (nothing to do with sea rise just stupidity).

gregk :

09 Feb 2011 7:51:08pm

We have..... "There are two principal reasons for rising sea levels. Firstly, the ocean is the Earth's thermometer — as the ocean warms as a result of increasing greenhouse gas concentrations in the atmosphere, it expands. Secondly, a warming atmosphere and ocean results in the melting of ice on land and the more rapid sliding of some glaciers into the ocean leading to a further increase of ocean volume".

And we have........."In many locations around the world there has been a significant increase in the frequency of high coastal sea levels. On both the east and west coasts of Australia there has been an increase in the frequency of high sea levels by about a factor of three during the 20th century".

Presumably therefore during the twentieth century we had glaciers sliding in to and out of the ocean and the oceans expanding and contracting at an increasing frequency.

John of Perth :

10 Feb 2011 1:17:05am

Following Tracy, I was part of some discussions in Darwin re coastal storm surges and re building. I was intrigued by some comments by a QLD person re the variability of many sandy water fronts moved in and out by considerable distances (+-150 m or so) depending on storms etc.

Very small differences in the energy arriving at the shore though increases sea levels will be reflected in shore line changes. Probably with an exponential relationship.

"Smoking does not cause Lung Cancer" has been morphed into "Man made pollution is not changing the climate".

Mike Pope :

11 Feb 2011 10:47:48am

Please Hawkeye! Spare us the pseudo-science and consider the following empirical facts that:

1. Since 1750, anthropomorphic CO2 has increased by over 38% from 280 ppm to 391 ppm and is continuing to increase.2. As a result average global temperature has risen by 0.7C since 1900 and is continuing to rise at an accelerating rate.3. Rising temperature is causing sea surface temperature to rise causing expansion of seawater.4. Rising temperature is causing land based glaciers and ice sheets to melt adding significant quantities of water to the oceans.5. These effects have resulted in average global sea level now rising by 3.2mm/year and continuing to rise at an accelerating rate.6. Based on on-going warming and ice melt, estimates are that sea level will rise by 1.5-2.5m by 2100.

Rise in sea level of this magnitude will affect for the vast number of people (over 70% of the world population) who live on or near the coast or rely on food grown on low lying fertile coastal plains and river deltas which are subject to flooding. Risk of flooding will be heightened if increased sea level is accompanied by a storm surge.

No one should be under the illusion that sea level is not rising and doing so at a speed which poses a serious and major threat to humans and the food supply on which they rely.

No one should doubt that CO2 is causing global warming which is melting ice, adding to sea level rise, that this will prove catastrophic before 2100 and that sea level will continue rising for hundreds of years.

John Church is of course quite correct in what he has to say though one suspects a trifle naive in thinking that populations have the will or capacity to adapt to rising sea level. Many lack the resources and technology are too stupid to do so.

The evidence for this is provided by those who operate assets or live on flood plains or in the path of cyclones, who have been repeatedly flooded or blown away. They persist in rebuilding in exactly the same location, without adequate protection against the elements. They do so knowing that in the future the same loss from flooding and wind will occur again and is likely to be more severe.

Henry Justice :

Sumner Berg :

14 Mar 2012 7:05:05pm

Sea levels rising ; sea levels falling! What's it really all matter when every year there are 80 million more humans stomping around on the globe. Maybe the mass of humanity will cause the continets to sink and cause ocean levels to rise!!Ha Ha!!